Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
Description of Related Art
In image forming apparatuses such as copier, printer, multifunction printer and the like, fixing devices for fixing not-yet-fixed images formed by toner onto recording members are used. With regards to such fixing devices, images are fixed onto recording members by forming fixing nips by sandwiching and conveying recording members by fixing rotation members equipped with heating units such as halogen heaters inside thereof, images being already formed on the recording members. As for the fixing rotation members, a pair of rollers including a heating roller equipped with a heating unit and a pressing roller, a device formed by winding a fixing belt around a plurality of rollers, and the like are used.
In a fixing device, fixing conditions suitable to various recording members are set in order to adapt to various types of recording members. For example, in order to fix a toner image onto a thick paper with heavy basis weight, there is a need to increase heat supply to the thick paper whose heat capacity is large. Therefore, fixing to such thick paper is assured by increasing the fixing temperature.
As for a technique relating to fixing device, JP 2012-8487A suggests a configuration including a tension roller which gives tension to a fixing belt which is wound around a heating roller so as to stabilize the distance between the heating mechanism, which heats the fixing belt and the heating roller, and the fixing belt. With such configuration, a preferable fixing of toner images can be realized.
However, there is a large difference in set fixing temperatures for thick paper whose heat capacity is large and thin paper whose heat capacity is small. Therefore, in a case of a job that requires switching between different paper types, there has been a problem that the productivity of printed documents decreases due to the time waiting for switching to the fixing temperature suited for thick paper from the fixing temperature suited for thin paper and vice versa being long.
In order to shorten the time waiting for switching the fixing temperature, the difference in the fixing temperature needs to be made small and the fixing nip width needs to be made large to make the heating amount accord with the conventional fixing condition when the fixing temperature of thick paper is to be decreased so as to approach the fixing temperature of thin paper. However, even if it is desired to increase the load in order to widen the fixing nip width, the fixing nip width cannot be widened since the change amount of the nip width is already at its utmost limit and cannot be widened anymore. Therefore, in order to make the difference in fixing temperatures be small, the fixing temperature of thin paper needs to be increased so as to approach the fixing temperature of thick paper. In order to increase the fixing temperature of thin paper, the nip width needs to be made small by decreasing the load so that the heating amount accord with the conventional fixing condition. However, if the load is decreased, the curvature of the fixing belt near the exit side of the nip is to be small as a side effect and the separability deteriorates.
Deterioration in separability due to decreasing the load occurs according to the following mechanism. FIG. 6A shows a configuration of a fixing device when a piece of thick paper is to be fed. FIG. 6B is an enlarged view of the part near the exit side of the nip in the fixing device when a piece of thick paper is to be fed.
The fixing device 120 includes a heating roller 130, an upper pressing roller 140, an endless fixing belt 150, an inlet roller 160 and a lower pressing roller 170. The upper pressing roller 140 includes an elastic layer 141 formed of a heat-resistant rubber or the like, for example. The heating roller 130, the lower pressing roller 170 and the inlet roller 160 are arranged by having predetermined distances therebetween. The fixing belt 150 is wound around the heating roller 130, the upper pressing roller 140 and the inlet roller 160. The lower pressing roller 170 is arranged so as to press against the fixing belt 150 in the region where the fixing belt 150 and the upper pressing roller 140 are in contact with each other. At the part where the fixing belt 150 and the lower pressing roller 170 are in contact with each other, the fixing nip 180 is formed in an upward convex shape.
At the fixing nip 180, the lower pressing roller 170 presses against the elastic layer 141 of the upper pressing roller 140 so as to dent. As shown in FIG. 6B, a part 142 (hereinafter, referred to as “bulged part”) which is formed in a downward convex shape having a large curvature is formed near the end on the exit side of the fixing nip 180 with respect to the recording member conveyance direction, the bulged part being a deflection in the elastic layer 141 of the upper pressing roller caused by the pressing of the lower pressing roller 170. The fixing belt 150 near the end on the exit side of the fixing nip 180 is tightly attached along the downward shape of the bulged part 142 and is formed in a shape having a large curvature that enables curvature separation. Therefore, such configuration is effective in terms of separability.
On the other hand, FIG. 7A shows a configuration of the fixing device when a piece of thin paper is to be fed and FIG. 7B is an enlarged view of a part near the exit side of the nip in the fixing device when a piece of thin paper is to be fed. As shown in an enlarged view of the bulged part 142 in FIG. 7B, when switching to the fixing temperature of thick paper to the fixing temperature of thin paper, if the width of the fixing nip 180 is made small by decreasing the load, the pressure from the lower pressing roller 170 is weakened and the downward convex shape of the bulged part 142 becomes small. Therefore, the curvature of the fixing belt 150 becomes smaller so as to return to the original circumferential curvature of the upper pressing roller 140 and the separability deteriorates.